Implement the get_variation_glyphs method for FreeType >= 2.3.6

Implement the get_variation_glyphs method for FreeType >= 2.3.6

* configure.ac: Check availability of FT_Face_GetCharVariantIndex.

* src/ftfont.c (ftfont_variation_glyphs) [HAVE_FT_FACE_GETCHARVARIANTINDEX]:
Add function implementation.

* src/ftcrfont.c (ftcrfont_variation_glyphs)
[HAVE_FT_FACE_GETCHARVARIANTINDEX]: Use function implementation for
HAVE_OTF_GET_VARIATION_GLYPHS.

* src/ftfont.c (ftfont_driver) [HAVE_FT_FACE_GETCHARVARIANTINDEX]:
* src/xftfont.c (xftfont_driver) [HAVE_FT_FACE_GETCHARVARIANTINDEX]:
* src/ftxfont.c (ftxfont_driver) [HAVE_FT_FACE_GETCHARVARIANTINDEX]:
* src/ftcrfont.c (ftcrfont_driver) [HAVE_FT_FACE_GETCHARVARIANTINDEX]:
Populate the get_variation_glyphs method.